1. Field of the Invention.
The present invention relates to an improved thin film platinum resistance thermometer, and to methods of fabrication of the improved thermometer. In particular, the present invention provides improved protection for a thin film platinum resistance thermometer to permit operation of the thermometer at elevated temperatures.
2. Description of the Prior Art.
For many years, platinum wire-wound resistors have been widely used for temperature measurements. In recent years, increased attention has been directed to thin film platinum resistance thermometers, which offer a number of unique advantages over wire-wound resistance thermometers.
First, thin film platinum resistance thermometers (TFPRTs) offer the capability of batch fabrication using techniques generally similar to those used in the semiconductor industry. Second, TFPRTs offer much greater flexibility in configuration. Third, TFPRTs offer the capability of extrmely small size. In contrast, demands for smaller dimensions typically require platinum wire wound resistors to use smaller diameter wire, and thus increase the difficulties in handling and manufacturing the resistor using such wire. Fourth, very precise trimming of the resistance values of a TFPRT potentially is possible using, for example, laser trimming.
The potential advantages of TFPRTs are discussed further in an article by W. Diehl entitled "Thin-Film-PRTD Platinum Thin-Film Resistors as Accurate and Stable Temperature Sensors", Measurements & Control, pp. 155-159 (December, 1982). Patents and patent applications describing TFPRTs include U.S. Pat. No. 4,050,052 issued to Reichelt; U.S. Pat. No. 4,072,593 issued to Diehl et al; U.S. Pat. No. 4,103,275 issued to Diehl et al; U.S. Pat. No. 4,129,848 issued to Frank et al; and United Kingdom patent application No. 2,096,645 published Oct. 20, 1982.
In two copending patent applications (Ser. No. 636,142 filed July 31, 1984 entitled "Platinum Resistance Thermometer", and Ser. No. 636,141 filed July 31, 1984 entitled "Method for Forming a Platinum Resistance Thermometer") which are assigned to the same assignee as the present application, an improved method of fabricating TFPRTs is described. The TFPRTs taught in these two copending applications are formed by a process in which an inert material such as silicone dioxide is deposited on a substrate and is selectively etched to form a negative pattern (or path) for the platinum thin film resistance element. The platinum thin film is deposited over both the exposed substrate surface and the surfaces of the inert material. The inert material is then etched away to permit the platinum which has been deposited on top of the inert material to be removed. This leaves only the platinum in the path which had been defined by the inert material. The resulting platinum resistance element has a columnar grain pattern. The sharp definition of edges of the resistance element, the columnar grain structure, and the minimizing of impurities results in much greater control of the temperature coefficient of resistance of the resistance element.
TFPRTs fabricated according to the method described in the two copending patent applications have exhibited excellent performance at temperatures below about 400.degree. C. At higher temperatures, however, degradation of the electrical characteristic has been observed. In order to permit practical application of TFPRTs to harsh environments and high temperature operation, an improved TFPRT structure is needed.